C programming two-hop neighbors - c

Question

I am not sure how to get my two-hop neighbors correctly. It's almost correct but on my output, I don't want to include the same vertex. For my output now if vertex 0 is 0, it says "vertex 0: 0.....
I want to skip the vertex it is currently looking at.
Please help me, are my codes for two hop wrong?
this is my codes:
#include<stdio.h>
#include<math.h>
#include<stdlib.h>
#include<time.h>
#define M 20
#define N 20
int main()
{
int i, j, x, a, b;
int G[20][20] = { { 0 } };
/*creaate random adjaceney matrix*/
printf("==================================================\n");
printf("Welcome to my Graph Processing tool!\n\n");
srand(time(NULL));
for (i = 0; i < M; i++) {
for (j = 0; j < N; j++) {
if (i == j) {
G[i][j] = 0;
}
else {
G[i][j] = rand() % 2;
G[j][i] = G[i][j];
}
}
}
/*check whether the whole row equals to 0*/
for (j = 0; j < N; j++) {
if (G[j] == 0) {
x = rand() % 20 + 1;
G[x][j] = G[j][x] = 1;
}
/*print the matrix G*/
else
{
printf("The adjacency for graph G is\n");
for (i = 0; i < M; i++) {
for (j = 0; j < N; j++) {
printf("%d ", G[i][j]);
}
printf("\n");
}
}
}
/*all one-hop neighbors*/
printf("\nList of one-hop neighbors:");
for (i = 0; i < M; i++) {
printf("\nVertex %d: ", i);
for (j = 0; j < N; j++) {
if (G[i][j] == 1) {
printf("%d ", j);
}
}
}
printf("\n===================================\n\n");
/*two-hop neighbors*/
for (i = 0; i < M; i++) {
printf("\nVertex %d: ", i);
for (j = 0; j < N; j++) {
if (G[i][j] == 0) {
printf("%d ", j);
}
}
}
}
printf("\n============================================\n");
system("pause");
return 0;
}
This is my output:
One hop
Two hop

Answer 1

The answer provided by #Jake only works for node 0. For only looking at different nodes you need to do
for (j = 0; j < N; j++) {
if (i != j && G[i][j] == 0) {
printf("%d ", j);
}
}
Furthermore, you are assuming that all nodes without an edge are two-hop neighbors. This is not correct. One way to calculate the actual two-hop neighbors would be ((A + I)^2 > 0) - ((A + I) > 0), where I is the identity matrix.
Also, you can code this via a three-layer loop:
int node, neighbor, neighbor2;
for (node = 0; node < N; node++) {
printf("\nVertex %d: ", node);
for (neighbor = 0; neighbor < N; neighbor++) {
if (G[node][neighbor] == 1) {
for (neighbor2 = 0; neighbor2 < N; neighbor2++) {
if (node != neighbor2 && G[neighbor][neighbor2] == 1) {
printf("%d ", neighbor2);
}
}
}
}
}
Note that per definition M=N, so I've just used N. Also, this might print some 2-hop neighbors twice. You might want to do some filtering before printing.

Answer 2

Couple things to note here.
Be more descriptive with your variable naming, it would have made this a lot easier to read.
M-ROWS, N-COLS, G-Graph
When you loop through each row, you initialize j to 0. This includes the vertex that you are wanting to leave out.
for (j = 1; j < N; j++)